The Afon Crawnon is a river in the Brecon Beacons National Park in the county of Powys

Powys

Powys is a local-government county and preserved county in Wales.-Geography:Powys covers the historic counties of Montgomeryshire and Radnorshire, most of Brecknockshire , and a small part of Denbighshire — an area of 5,179 km², making it the largest county in Wales by land area.It is...

, south Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

. The headwaters rise on the hills north of Trefil

Trefil

Trefil is a small village in the county borough of Blaenau Gwent, south Wales. It lies at the top of the Sirhowy Valley, three miles northwest of Tredegar...

 and flow northeast down a valley known as Dyffryn Crawnon for 7km to a confluence with the River Usk

River Usk

The River Usk rises on the northern slopes of the Black Mountain of mid-Wales, in the easternmost part of the Brecon Beacons National Park. Initially it flows north into Usk Reservoir, then east by Sennybridge to Brecon before turning southeast to flow by Talybont-on-Usk, Crickhowell and...

 just west of Llangynidr

Llangynidr

Llangynidr is a village in Powys, Wales, about west of Crickhowell and south-east of Brecon. The River Usk flows through the village as does the Monmouthshire and Brecon Canal on which there are four locks....

. The valley is notable for being at the centre of a major green energy initiative known as The Three Green Valleys project and which involves the construction of several micro-hydro schemes.Dyffryn Crawnon is translated from Welsh to mean "Valley of the Garlic River".

Geology

The river flows through a valley carved into the mudstones and sandstones of the Old Red Sandstone

Old Red Sandstone

The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...

 laid down during the Devonian period.

Access

The public road up the valley is effectively a cul-de-sac though numerous footpaths and bridleways climb up the sides of the valley from it. However, a vehicular impassable Restricted Byway (Road used as Public Path) connects to an equally difficult in law Right of way at the end of the valley, suspected to be the original line of the Brinore Tramroad. The Beacons Way skirts the northern edge of Dyffryn Crawnon.